Evaluation of formula field dissapears after updating related field using the API

Evaluation of formula field dissapears after updating related field using the API

We have a custom application interfacing with Zoho CRM through its API. This application runs daily and updates some custom fields within Zoho from external sources.

There are some formula fields within the jobs area that use some of the fields that we are updating through the API. 

For example, 2 of the custom fields we update on a job are "Remaining hrs estimated" and "Hours logged". Then there is a custom formula field called "Projected Hours" which has the following formula:

${Potentials.Remaining hrs estimated}+${Potentials.Hours logged}

The problem is that every time the application is run and these fields are updated, the evaluated value of the formula fields disappear when viewing that job. 

They can be brought back by editing the job, and saving it again, which is not ideal.

I have noticed that when editing a formula field in the admin section, I receive a warning that states " Formula custom fields created or modified will not be evaluated in already existing records."
I would therefore understand if the old evaluated value remained, but instead it disappears altogether.

Could somebody let me know whether this is a bug, intended functionality, or whether some required action has been missed?

Thanks in advance,

Simon R